Co-extrusion through a series of layer multiplying die elements enables the production of films containing hundreds and even thousands of layers, with individual layer thickness varying from the microscale to the nanoscale. This results in the production of novel systems with improved properties. In fact, layered polymer films with micron and sub-micron-thick layers have seen increasing usage in composite films for packaging with improved gas barrier and increased mechanical toughness. Commercialized technologies stemming from layered polymer films with unique optical properties include highly reflective light filters and polarizers.
One exemplary layer multiplying die currently used was developed by The Dow Chemical Company of Midland, Mich. Later optimizations of the Dow design as well as polymer selection via rheology matching and incorporation of polymeric surface layers has led to improvements in layer uniformity. However, even after the more recent optimizations of the layer multiplier die design, the layer uniformity and continuity strongly depends on the viscosities of the individual layered components. Poor viscosity matching normally results in the lower viscosity polymer encapsulating the other by forming a slip film between the higher viscosity polymer and the multiplier die wall. It is also known that normal stress differences can cause layer instability and breakup during layer multiplication. The above means that currently good layer uniformity and continuity can only be achieved for a relatively low range of materials. In general, the current layer multiplying dies cannot be used to obtain products with enhanced properties when the differences between the viscosities of the individual layered components and/or their elasticities are relatively high.
In the present invention, a new multiplier die is provided that is capable of broadening the range of polymers that can be layered in order to obtain products with enhanced properties. This new multiplier die allows for film production with good layer uniformity and continuity even when the differences between the viscosities and/or polymer elasticities are relatively high.